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Rare & Remarkable Human Traits

Tetrachromacy: Can Some Women Really See 100 Million Colors?

Some women may carry a fourth color cone, but the 100 million colors figure is an estimate. Here's what studies have and haven't shown.

By Written Facts StaffOctober 14, 20267 min read

You're walking under a maple in October, and the leaves have gone a dozen kinds of orange, red and gold. Now imagine someone beside you seeing far more shades in the same tree. Can some women really see 100 million colors? Some women may have the eye hardware for it: a fourth type of color-sensing cone. But "100 million" is an estimate from arithmetic, not a measured result. Very few women have shown in a lab that they can actually use that extra cone.

How Normal Color Vision Works

Most people see color with three types of cone cells in the retina. According to Discover and the University of Washington's ophthalmology department, each cone type can tell apart about 100 shades. Combined, that lets a typical "trichromat" distinguish roughly 1 million colors.

The same math works in the other direction. A dichromat, with only two cone types, distinguishes about 10,000 combinations, and Discover notes that most mammals, dogs included, are dichromats.

Where the 100 Million Figure Comes From

Add a fourth cone and you get the headline number. Jay Neitz, Bishop Professor of Ophthalmology at the University of Washington, has said tetrachromats can see about 100 million colors (UW Ophthalmology).

The figure is a theoretical estimate: roughly 100 shades per cone, multiplied across four cones (100 × 100 × 100 × 100). In 2014, Popular Science reported that researchers had not verified it by experiment, because standard color tests aren't designed for people with more than three pigments. So the 100 million figure shows what four cones could make possible. No one has measured anyone actually seeing that many colors.

Why It's Mostly About Women

In 1948 the Dutch scientist HL de Vries noticed something about the mothers and daughters of color-blind men: they could carry four cone types instead of three, and he suggested they might see more colors (ScienceAlert; Discover).

The reason is genetic. The trait is carried on the X chromosome (UW Ophthalmology), and women have two X chromosomes. A woman can inherit a normal cone gene on one X and a slightly shifted version on the other. That can give her a fourth cone tuned between two of the usual three.

The number most often quoted comes from research by John Mollon and Gabriele Jordan: about 12% of women carry a mild X-linked color-vision variation, which makes them possible four-cone candidates (ScienceAlert; Discover). Other estimates vary a lot. Wikipedia cites one study suggesting about 15% of women and another suggesting as many as 50% may have four photopigments. The honest answer is that nobody knows the exact share.

Having Four Cones Is Not the Same as Seeing More

This is the main point of the topic. Having a fourth cone doesn't mean a woman sees extra colors. According to Wikipedia's summary of the research, many studies have confirmed the fourth cone in women, but enhanced color discrimination, called "functional" or "behavioral" tetrachromacy, has been documented in only one case in the scientific literature.

Mollon, at Cambridge, began looking for tetrachromats in the late 1980s. His early tests asked mothers of color-blind sons to match colors. If those women truly saw an extra dimension of color, some matches should have looked wrong to them. Instead, they could always make the match, just as a trichromat would (ScienceAlert; Discover).

Jordan, at Newcastle University, put it plainly: "we don't know what allows someone to become functionally tetrachromatic, when most four-coned women aren't" (Wikipedia). A 2019 review by Jordan and Mollon noted that public interest in tetrachromacy has grown while rigorous studies remain sparse (Newcastle University eprints).

The Woman Known as cDa29

The best-known case comes from Jordan's lab. In 2007 she tested 25 women who had a fourth cone. On each trial, three circles of light flashed, and one of them held a subtle color mixture that a normal trichromat couldn't tell apart from the others. One woman, code-named cDa29, picked the right circle every time (ScienceAlert; Discover). Discover describes her as a doctor in northern England.

The result needs some caution. As of ScienceAlert's 2016 report, the cDa29 research had not been peer-reviewed or published, still needed replication, and what she actually sees remained unknown. Passing a hard color test shows she can detect differences other people miss. It doesn't tell anyone what those colors look like to her.

Concetta Antico, the Painter

The other name that comes up often is Concetta Antico, an impressionist-style painter. Popular Science reports that researchers identified her as a tetrachromat in 2012. She was studied by Kimberly Jameson of UC Irvine and Alissa Winkler of the University of Nevada, Reno. Jameson's genetic analysis suggests Antico's fourth cone responds to reddish-orange-yellow wavelengths, but what she perceives through it is uncertain.

A 2016 paper by Jameson and colleagues tested six women, four of them potential tetrachromats. According to a summary of the study, three of the four showed no color discrimination beyond normal human variation. Antico outperformed everyone, especially on reds and in low light, which supported her status as a functional tetrachromat. The sample was very small, though, and larger studies may not confirm the result. Even here, the pattern matches the rest of the research: most four-cone women performed like everyone else.

Why the Extra Cone Might Sit Unused

If the hardware is there, why don't more women use it? Neitz has suggested the human world is "tuned to the world of the trichromat" (ScienceAlert; Discover). Paints, dyes, screens and printed colors are made for three-cone eyes. A potential tetrachromat may need practice, or specially designed colors, before the fourth cone makes a difference. That could explain why an artist who spends years studying subtle color would stand out. It remains an idea, though, not a proven explanation.

The research also has gaps. ScienceAlert notes that the cDa29 study tested only women in the UK, and most four-cone women may never use their fourth cone at all.

So, Can Some Women Really See 100 Million Colors?

Here's what's known:

The fourth cone is real. Many studies have confirmed that some women carry it, and UW Ophthalmology notes that a genetic test exists to check for it.
How many women carry it is uncertain. Estimates range from about 12% to as high as 50%.
Using it is rare. Only a tiny number of women, such as cDa29 and Concetta Antico, have shown better color discrimination in testing, and their results come from small studies.
"100 million" is an estimate. It's what four cones could theoretically support. Nobody has measured it.

So the next time a maple turns color this fall, the person beside you probably sees about what you do. The fourth cone is real, and a few women may get more out of it than the rest of us. How much more is still an open question.

Sources (7)

Photo by athree23 on Pixabay.